This invention relates to electrical circuits for detecting the status of a switch and more particularly, to an isolating circuit for use in a microprocessor based control system for detecting the state of a switch connected to a power line.
Microprocessors are being increasingly used in control units for household appliances. As such, they are replacing mechanical logic elements and their associated wiring which, up till now, carried out necessary control functions in the appliance. In a washing machine, for example, the control of the machine cycle is based upon a logical sequence of events whose completion must be sensed in order for the machine to perform its job. The circuitry involved typically includes at least one water level switch, since only when the water reaches a predetermined volume (a minimum value) should the water heating elements be activated. Consequently, the water heating elements include a relay which is series connected with the water level sensor. Similarly, the motor turning the washing machine tub should not turn on unless the door or lid for the tub is closed. Thus the power switch for the motor is series connected with a switch or relay which is activated upon the door or lid being closed.
Introduction of microprocessor control schemes has obviated the need for much of the complex wiring and mechanical elements previously required to make the appliance function properly. Rather than, for example, the timer motors and mechanical switches previously used, a microprocessor chip is programmed to make the various operating decisions required during the machine's work cycle. To do this, however, the microprocessor still must receive as inputs information representing the current status of certain machine components. In the washing machine example noted above, line connected switches are now used. These are switches designed for operation at line voltage (i.e. 110 Vac, 230 Vac in Europe) connected in series with other components for safety reasons. The problem with using these switches is first, detecting the operating status of the switch, which is subject to a high voltage level, and providing a status indication to the microprocessor which operates at a much lower voltage level. Subjecting the microprocessor to any voltage approximating line voltage would destroy the microprocessor. Second, the switch status sensing must conform to the topology of the appliance; i.e. it must not interfere with the operation of other machine circuits.